A process of this kind and a corresponding apparatus for it have been disclosed by DE-PS 26 14 258.
Air tunnels of this kind are generally 1,000 mm wide and up to 5,000 mm long and are used for thermal processing of monofilaments. They are configured in two parts, and the monofilaments travel through the air tunnel between the two halves of the unit. To achieve uniform physical properties--such as tensile strength, elongation, modulus of elasticity, shrinkage, dimensional stability, abrasion resistance, and flexibility--for each of the individual monofilaments of a bundle, not only must the temperature over the entire surface constituted by the bundle of monofilaments be uniform, but also deviations in air velocity over the width and length of the air tunnel must be negligibly small. With air tunnels considered to embody the state of the art, temperature tolerances of T=.+-.1.degree. Celsius and air velocity tolerances of v=.+-.0.05 m/sec can be achieved in the working duct for the monofilaments.
It is easy to understand that the tolerances indicated can be achieved, over a surface area of as much as F=5 m.sup.2, only with very costly equipment, i.e. the walls must be especially thick, seals must be selected with particular care, and the outlay for manufacturing equipment surfaces that are subject to large temperature fluctuations, at least when a system is started up and shut down, is especially high.
Furthermore, in known air tunnels that are operated exclusively with hot air a maximum production volume is already being reached today; however this is no longer adequate for the demands of modern production lines, which are operating at higher and higher speeds.
The known air tunnel that is operated with hot air has the disadvantage of requiring long temperature compensation sections in order to heat the large masses of equipment uniformly. This results in a long warmup time when the known air tunnel is started up. Hot air is guided in two unit halves which are pivoted with respect to one another, the air channels of which are connected to each other. This connection is made by means of bellows or specially configured stuffing boxes. This design solution is complex and prone to malfunction.
Furthermore, the air delivery nozzles pull cold air into the known air tunnel. This has a detrimental effect on the surface temperature profile. The temperature profile is also subject to increased fluctuations in the region where hot air is withdrawn from the air tunnel, and at the lateral surfaces. The effect of these temperature changes is to reduce the quality of the monofilaments which are passing through, thus detracting from the uniformity of the material properties of the monofilament bundle. In addition, transverse baffles in the known air tunnel generate turbulence, which interferes with a uniform air velocity over the width and length of the known air tunnel. Lastly, mention should also be made of the numerous regulating flaps that are required in the known air tunnel so that air volumes can be distributed approximately uniformly.
The object on which the invention is based is therefore that of developing an air tunnel of the aforesaid type in such a way that with highly accurate temperature control, the stress on the individual monofilament in the bundle is uniform and homogeneous.